Anti-lock brake systems (ABS) for air braked truck trailers are in common use and generally include an Electronic Control Module (ECM), Pneumatic Control Module (PCM) and wheel speed sensors. The various components may be known by different names, for example the ECM is sometimes called the Electronic Control Unit (ECU). The PCM may be known as a modulator. Some ABS implementations integrate the PCM into another component known as the relay valve. In this case the pneumatic component is sometimes called a modulating relay valve.
The wheel speed sensors provide wheel speed information to the ECM and the ECM signals the PCM to modify air pressure level at the brake chambers. This process controls the braking level so that the wheels continue to rotate. The overall process is described in detail in numerous patents and in the pending U.S. patent application Ser. No. 09/306,921.
On air-braked truck trailers in use today, a warning lamp is provided which normally goes through a bulb check sequence at startup, but otherwise illuminates only if there is a problem. The main purpose of the warning lamp is to alert the driver and/or maintenance personnel to an ABS fault. The driver can then drive more cautiously until the ABS is repaired. To avoid driver confusion, the warning lamp does not normally provide any kind of troubleshooting information which might be helpful in repairing the ABS. The warning lamp is strictly a go/no go indicator.
The warning lamp is subject to regulation in the United States on trailers. In the United States, the warning lamp is required to be at a particular location towards the left rear of the trailer and it is not normally visible from the cab of the tractor. The driver or maintenance person has to walk by the left rear of the trailer for a periodic inspection of the warning lamp.
When an ABS failure does occur, maintenance personnel need to troubleshoot the ABS so that the faulty components can be repaired or replaced. The warning lamp normally provides little help as it is purposely designed to provide a simple failure indication. To avoid driver confusion, detailed troubleshooting information is not normally provided.
Various manufacturers have approached the diagnostic problem in similar ways, but with subtle differences.
A trailer ABS marketed in North America by Bosch/Eaton sometimes includes an additional 2-wire cable which puts the ECM and warning lamp into a diagnostic mode when the two pins are shorted together. Depending on the fault, a series of flash codes is output by the warning lamp to identify the particular problem. Driver confusion is not an issue as this is a special mode of operation which cannot occur without deliberate action by maintenance personnel.
Wabco uses a mechanism where a particular power-up sequence puts the ABS into a diagnostic mode. This relies on the fact that for ABS on North American trailers, the ECM receives electrical power from the stop lamp supply line and from an auxiliary electrical supply line which is intended to have power at all times when the ignition in the tractor is switched on, see FIG. 1. The stop lamp supply line is only powered when the brakes are applied by the operator. The auxiliary supply line is a better source of power but when the trailer is coupled to an older tractor, the auxiliary supply line may not be available. Hence, the requirement for stop lamp power is considered to be a backup. The Wabco diagnostic approach requires that the trailer first be coupled to a suitable tractor which can power the auxiliary supply line. The brakes are applied without turning on the ignition. Thereafter, the ignition is switched on and off a specified number of times to induce a particular diagnostic mode. For example, the display of failures which currently exist may constitute one diagnostic mode. The display of failures which previously existed but are no longer present may constitute another diagnostic mode. A significant disadvantage with this approach is that it is a two-person operation. An operator sits in the tractor and operates the brakes and ignition key while an observer at the rear of the trailer notes warning lamp behavior.
Another approach which is available with most existing systems is to use a diagnostic data link so that the ECM communicates with a separate diagnostic computer in the trailer maintenance shop. This may be a general purpose computer running a special program or it may be a dedicated piece of test equipment. This technique can provide very precise diagnostic information. A disadvantage is that the diagnostic computer is required. Also, connecting the computer to the vehicle is also not a trivial task under the conditions which prevail in the trailer maintenance shop. Another disadvantage is that, from an economic and design perspective, a special harness and connector is required on every trailer to provide for the computer hook-up to the trailer. A high quality harness and connector are required to ensure that reliable connections can be made after many miles of service in an over the road environment. This harness and connector can be a significant portion of the overall system cost.
An ECM for an ABS is normally designed to tolerate all, or almost all, possible miswiring possibilities. This includes the possibility of a short across the warning lamp. The occurrence of such a short on some portion of the trailer population can reasonably be expected as the wire from the ECM to the warning lamp often follows a long and tortuous path through the trailer. On most North American trailers, one side of the warning lamp is permanently connected to ground (to the chassis) while the ECM provides the switched 12-volt supply to the warning lamp. This arrangement allows for the use of only one wire from the ECM to the warning lamp and a short connection from the warning lamp to a convenient grounding point on the chassis. If the wire gets pinched anywhere as the wire is routed through the chassis, then the 12-volt feed is shorted to ground. Most electronic components used as switches would be destroyed in such circumstances. The ECM of an ABS, however, is generally designed to tolerate such abuse. When the short is removed the ECM will work satisfactorily and will again illuminate the warning lamp as originally designed.
The electronic circuitry which implements this short circuit protection requires significant complexity. If electric current is simply limited to a value which is safe for the component involved, then heat dissipation in that component and in the ECM becomes significant. For example if a one amp limit is chosen, then for the twelve volt systems used in North America, heat dissipation is approximately twelve watts. With suitable heat sinking, this can be handled. However, substantial cost and complexity have to be added to the ECM which may not otherwise require any heat sinking over and above the printed circuit board itself. Cost and complexity of the final assembly operation for the ABS is also increased. If no heat sinking is required, then final assembly can consist of simply putting a printed circuit board in a suitable box. If heat sinking is required, electronic components my have to be attached mechanically to the metal box and electrically connected to the printed circuit board. This adds assembly cost, introduces tolerance difficulties and may also detract from durability under the harsh shock and vibration environments encountered on a trailer.
Some designs of the ECM avoid the use of heat sinking by recognizing that a short exists and then reducing the current to a level close to zero, basically turning off the drive. This recognition requires either that there be complex analog and/or digital circuitry in the design or that the increase in current is somehow communicated to a microprocessor or micro controller which then turns off the drive under software control. Typically this latter approach requires that relatively simple analog circuitry limit the current to some appropriate initial level. This provides protection for the device for a short time (of the order of milliseconds) in that its current carrying capacity is not exceeded. However, the power dissipation is perhaps an order of magnitude higher than the device can survive under steady state conditions. The micro controller then determines that this condition exists and turns off the device before the enormous dissipation has elevated the device temperature to unsafe levels. This is the approach taken with current and anticipated future ABS products manufactured by the present Assignee.
For a warning lamp drive there is an additional level of complexity in that an incandescent bulb has a very low resistance when power is first applied. Consequently, the warning lamp has a large inrush current when turned on. As the filament warms up, resistance increases and the current reduces to the normal level. Any short detection mechanism must allow for the initial inrush current and allow current to flow at an acceptable level until the filament warms up.
The present invention utilizes the significant complexity just described to provide the additional function of a switched input signal to the ECM without the use of any additional wiring. This input signal is used to select different diagnostic modes for the ECM. The input signal can be used for other purposes. Other features and advantages of the present invention will become apparent upon reading the attached specification, in combination with a study of the drawings.
A general object of the present invention is to provide a novel system for selecting a diagnostic mode on a vehicle, such as a trailer.
An object of the present invention is to provide a novel diagnostic system for a vehicle, such as a trailer, which uses an existing warning lamp and existing wiring on the vehicle.
An object of the present invention is to provide a novel diagnostic system for a vehicle, such as a trailer, which communicates with a technician once a short is applied by the technician to the system.
Briefly, and in accordance with the foregoing, the present invention discloses a novel diagnostic system for a vehicle, such as a trailer. The system includes a micro controller which is part of an anti-lock brake system of the trailer. A warning light is provided on the trailer and is in electrical connection with the micro controller. Means for applying a short, such as a switch, is provided proximate to the warning lamp. The short applying means can be activated by a technician to provide a short across the warning lamp. A stored program within the micro controller recognizes the short and performs a diagnostic function in response thereto by flashing the warning lamp in a predetermined manner.